OK, the problem is now fixed and Windows sees 16 GB of RAM.
I ran MemTest as suggested and it reported no errors. Consulted the m/board manual and the 2 RAM sticks were indeed installed correctly, with 1 each side of the CPU in slot #1 (left of CPU) and #2 (right of CPU). Removed and re-installed the sticks into each others slot and the situation was unchanged, still only seeing 8 GB of RAM. Figuring I had nothing to lose, I removed 1 stick from the #2 slot and installed it into the #4 slot (outboard of #1). Two sticks on the LH side of the CPU and two empty slots on the RH side.
Bingo! Problem cured, 16 GB of RAM now active and the machine is working like it should.
Many thanks for your time and advice.